The present invention relates to a method for fabricating an electrophotographic recording material containing a double layer composed of amorphous and crystallized selenium applied to an electrically conductive substrate.
Electrophotographic copying methods and apparatus for practicing such methods are widely used in the duplicating art. They utilize the property of photoconductive materials to change its electric resistance when exposed to an activating radiation.
After imparting an electrical charge, a layer of such photoconductive material and exposing the photoconductive layer to an activating radiation in a pattern corresponding to an optical image, a latent electrical charge image can be produced thereon to correspond to the optical image. At the exposed locations, the conductivity of the photoconductive layer increases to such an extent that the electrical charge thereat can flow off, at least in part, but in any case to a greater extent than at the unexposed locations, through the conductive substrate, while at the unexposed locations the electrical charge remains essentially unchanged. The pattern of the subsisting charge can be made visible with an image powder, a so-called toner, and the resulting toner image, if required, can be transferred to paper or some other carrier.
Organic as well as inorganic substances are used as the electrophotographically active substances. Among them selenium, selenium alloys and compounds containing selenium have gained particular significance. They perform well, particularly when in the amorphous state, and have found manifold uses in practice.
The change in the electrical conductivity of a photoconductor depends on the intensity and the wavelength of the radiation employed. In the range of visible light, which is preferred for practical use in electrophotography, amorphous selenium exhibits high sensitivity on the blue side, i.e. in the short-wave range, while on the red side, or in the long-wave range, it exhibits only slight sensitivity.
This has the result that on an electrophotographic plate, a red character is reproduced exactly the same as a black character which under certain circumstances, particularly when the original is in color, has drawbacks for practical use; a black character on a red background, or vice versa, for example, will not be distinguishable from its background and can therefore not be identified.
It is known that crystallized selenium, in contrast to amorphous selenium, is extremely sensitive to red. Thus, with its use, the part of the visible spectrum above 650 nm can also be utilized. However, because of its high dark conductivity, i.e. its characteristic of being such a good conductor of electrical current even in the unexposed state that a charge applied to its surface cannot be maintained long enough for electrophotographic purposes, crystallized selenium has not been considered desirable for electrophotographic purposes.
A photoconductive material which is sensitive to red as well as blue and simultaneously is characterized by low dark conductivity, is a combination of amorphous and crystallized selenium. The two selenium forms may, for example, be in a layer structure in which first a layer of crystallized selenium is applied to a conductive substrate and then, thereupon, a layer of amorphous selenium.
The manufacture of such a double photoconductive layer is extremely difficult because, for perfect operation, i.e. particularly for uniform red sensitivity, it is necessary that the lower layer crystallize completely uniformly, and in always the same manner over the entire surface area of the photoconductor so that the layer completely covers the substrate and is of a uniform and sufficient thickness.
During conventional thermal treatment of a vapor-deposited amorphous layer during or after the vapor-deposition process in order to produce crystallization under the influence of higher temperatures, such uniformity can be produced only with difficulty, if at all. Firstly, a minimum temperature of about 70.degree. C. is required to produce crystallization, and secondly, the temperature should be kept as low as possible in order to produce the best possible uniformity of crystallization. For these reasons a narrow temperature range must be set during the vapor-deposition process and must be kept constant, which requires considerable expenditures particularly for large scale production, with the added impediment that the temperature regulation must take place in a vacuum.
Moreover, crystallization is dependent to a particular degree on the unpredictable presence of other crystal seeds such as surface inhomogeneities, which initiate spontaneous crystallization that is difficult to influence, and depends on the variables of the selenium batch as well as on the variables of the individual substrate. With the crystal seed formation already being so irregular, there can be no assurance that the further crystallization can proceed with the necessary uniformity.
If, on the other hand, the procedure is effected at a higher temperature in order to better assure that the crystallization of the lower partial layer is complete, there again exists the danger that the upper partial layer, which, in order to maintain its sensitivity range, must remain in the amorphous state, also changes more or less intensively and irregularly to the crystallized state, which is just as undesirable.
An article by Kohei Kiyota and Kunihiko Tasai, entitled "Selenium Element for Photo-Electrostatography", published in the periodical Fujitsu Scientific and Technical Journal, December, 1975, discloses that a photoconductive double layer of crystallized and amorphous selenium can be produced by vapor-depositing selenium onto a layer of manganese, the manganese acting as a crystallization seed for the selenium. The conversion of the initially amorphous selenium into the crystallized form is accomplished by subsequent tempering which takes place at about 80.degree. C. in order to attain a conversion speed which is high enough to cause a satisfactorily short conversion time to be required. The drawback of the selection of such a high conversion temperature is the above-described danger that the upper, amorphous partial layer then also changes to the crystallized state. Thus this process likewise requires the maintenance of a very closely defined temperature range. Moreover, after the vapor-deposition step there still is required an additional process step for tempering.